Fairing for marine drilling risers

ABSTRACT

A fairing includes two half-shells of fiber reinforced plastic or fiberglass. The shell halves are shaped to close around the buoyancy modules (e.g., cylindrical syntactic foam buoyancy modules) commonly used on deep-water risers, but with an extended trailing edge. The fairing includes a tail that streamlines the riser and reduces/prevents formation of the vortex, which causes drag and vibration. Hinges at the tail join the halves and enable them to be opened for assembly around the riser pipe as it is being run into the water. The fairing may be fastened together by bolted stainless steel tensioners that tightly draw together straps of high strength aramid fiber, such as KEVLAR®. The fairing is positioned on the riser such that the fairing is free to swivel and rotate into the current, while integral buoyancy blocks support the tail. To prevent movement up or down the riser string, the fairing is held in place by a channel clamp gripping the outside diameter of the syntactic foam buoyancy module.

PRIORITY INFORMATION

This application claims priority under 35 U.S.C. §119(e) from U.S.provisional application Ser. No. 60/834,219 filed Jul. 28, 2006, whichis hereby incorporated by reference.

BACKGROUND OF THE INVENTION

As offshore oil and gas exploration moves into deeper and moreinhospitable waters, the danger of equipment failure increases. One ofthe hazards now being encountered is that of high-speed (e.g., 3 knotsand greater) currents damaging the riser pipe that connects a floatingdrill rig to its wellhead on the ocean floor. Such currents exert amassive drag force on the pipe while causing it to shake violently in aphenomenon called vortex-induced vibration (VIV).

Fairings, strakes, shrouds, and similar devices have been used for manyyears in an attempt to prevent riser damage by currents. These deviceshave been only partly successful because they were (1) overly complexand expensive; (2) difficult and time-consuming to assemble on board therig; and (3) lack the necessary robustness to withstand the rigors ofshipboard use.

There is a need for a fairing that decreases drag and vortex inducedvibration to marine risers.

SUMMARY OF THE INVENTION

A fairing includes two half-shells of fiber reinforced plastic orfiberglass. The shell halves are shaped to close around the buoyancymodules (e.g., cylindrical syntactic foam buoyancy modules) commonlyused on deep-water risers, but with an extended trailing edge. Thefairing includes a tail that streamlines the riser and reduces/preventsformation of the vortex, which causes drag and vibration. Hinges at thetail join the halves and enable them to be opened for assembly aroundthe riser pipe as it is being run into the water. The fairing may befastened together by bolted stainless steel tensioners that tightly drawtogether straps of high strength aramid fiber, such as KEVLAR®. Thefairing is positioned on the riser such that the fairing is free toswivel and rotate into the current, while integral buoyancy blockssupport the tail. To prevent movement up or down the riser string, thefairing is held in place by a channel clamp gripping the outsidediameter of the syntactic foam buoyancy module.

A number of different clamp and fairing arrangements are of coursepossible. The half-shells may be molded. The integral buoyancy modulemay include syntactic foam.

Significantly, the channel clamp is secured in place around a buoyancymodule by a strap, such a KELVAR synthetic fiber strap. These straps maybe tightened securely in place by a tension rod. The tension rod mayinclude a thread bar and nut such that as the nut is run up the strap istightened to secure the clamp to the buoyancy module.

In one embodiment, the adjustable clamp may be mounted such that afairing is mounted around the clamp so the fairing weathervanes with thecurrent. The adjustable clamp may also be mounted externally to thefairing(s), for example to hold one or more fairing in place along thebuoyancy modules. For example, a plurality of fairings may be securelypositioned between first and second adjustable clamps.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of preferred embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a top view and several side views of the fairing;

FIG. 2 illustrates a perspective view of the fairing in the openposition; and

FIGS. 3A-3C illustrate several different fairing and clamp arrangementson a riser.

DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a top view and several side views of a fairing 10.The fairing 10 includes first and second fairing halves 12, 14 that arehinged and operatively secured together. The first and second fairinghalves 12, 14 are preferably constructed of fiberglass. The firstfairing half 12 includes a first sidewall 16 (e.g., semi-circular),while the second fairing half 14 includes a second sidewall 18 (e.g.,semi-circular). The first and second fairing halves 12, 14 include firstand second tails 20, 22 respectively, which extend from their associatedfairing half and are joined together at a hinge 24.

FIG. 2 illustrates a perspective view of the fairing 10 in the openposition. The fairing also includes first and second synthetic straps26, 28, such as for example KEVLAR straps that are used with associatedstainless steel tensioners and suitcase latches to securely close thefairing in an operable position around the buoyancy module. Strong andflexible straps of aramid fiber attach the fairings tightly, yet haveenough elasticity to accommodate riser motion under the most difficultconditions of bad weather and high current.

Buoyancy material 30, 32 such as syntactic foam may be placed alonginterior sidewalls of the first and second tails 20, 22 to increase thebuoyancy of the fairing.

To secure the fairing 10 to the riser buoyancy module an adjustablechannel clamp 40 (e.g., metallic such as aluminum) that includes firstand second halves 42, 44 is placed around the riser buoyancy materialand closed. The clamp is secured in the closed position with a syntheticfiber strap 46 (e.g., a KEVLAR strap). The strap 46 may be tightened bya tension rod to secure the channel clamp in place on the riser buoyancymodule. The tension rod may include a thread bar and nut, such that asthe nut is run up the strap 46 is tightened securing the clamp 40 to theriser buoyancy module. Advantageously, the strong and flexible straps ofaramid fiber attach the fairings tightly, yet have enough elasticity toaccommodate riser motion under difficult conditions of bad weather andhigh current.

The adjustable channel clamp 40 may be mounted such that the fairing 10rotates around to weathervane with the current, thus reducing vortexinduced vibration. As shown in FIGS. 1 and 2, exterior surfaces 50 ofthe clamp 40 rest within a channel 52 on the fairing 10 to allow thefairing to rotate with the ocean current.

FIGS. 3A-3C illustrate several different fairings and clamp arrangementson a riser. FIG. 3A illustrates a single clamp that may be used perfairing, with the clamp 40 being positioned within the channel of thefairing. FIG. 3A illustrates an embodiment comprising six fairings andfour clamps, where two fairings comprising clamps sandwich a fairingthat does not include a clamp. FIGS. 3A-3C illustrate an embodimentcomprising eight fairings and five clamps, where two fairing aresandwiched between a fairing mounted externally to the fairings.Advantageously, the adjustable channel clamp may be located within oroutside the fairing depending upon the fairing configuration beingemployed. In each embodiment the fairings are free to rotate with thecurrent.

Hydrodynamic testing indicate that fairings according to the presentinvention reduce drag on a marine riser by as much as 75% andsignificantly reduce/virtually eliminate the harmful effects of vortexinduced vibration.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

1. A fairing for use on with a riser, comprising: an adjustable channelclamp that includes first and second clamp halves that arelongitudinally and rotatably secured together around the riser with afirst synthetic fiber strap; a fairing that includes first and secondfairing halves secured around the riser, where the first and secondhalves form a channel around an interior periphery of the first andsecond halves such that the adjustable channel clamps rides within thechannel; and a second synthetic fiber strap that co-axially encirclesthe fairing to hold the first and second fairing halves in a closedposition.
 2. The fairing of claim 1, where the each of the first andsecond fairing halves includes a semi-cylindrical component thatpartially surrounds the riser and a tail that tapers downward to a pointat a tail end of the fairing.